I. Field of the Invention
The invention generally relates to mobile communication systems and in particular to techniques for activating a high frequency clock following a sleep period within a mobile station of a mobile communications system employing slotted paging.
II. Description of the Related Art
Certain state of the art wireless communication systems, such as Code Division Multiple Access (CDMA) Systems, employ slotted paging to allow mobile stations to conserve battery power. In a slotted paging mode, paging signals are transmitted from a base station to particular mobile stations only within assigned paging slots separated by predetermined intervals of time. Accordingly, each individual mobile station may remain within a sleep mode during the period of time between consecutive paging slots without risk of missed paging signals. Whether any particular mobile station can switch from an active-mode to a sleep mode depends, however, upon whether the mobile station is currently engaged in any user activity such as processing input commands entered by the user or processing a telephonic communication on behalf of the user. Assuming that the mobile station is not currently engaged in any processing on behalf of the user, the mobile station automatically powers down selected internal compounds during each period of time between consecutive slots. One example of a slotted paging system is disclosed in U.S. Pat. No. 5,392,287, entitled xe2x80x9cApparatus and Method for Reducing Power Consumption in a Mobile Receiverxe2x80x9d, issued Feb. 21, 1995, assigned to the assignee of the present invention and incorporated by reference herein.
Thus, within the slotted paging mode, a mobile station reduces power consumption by disconnecting power from selected internal components during a sleep period between consecutive slots. However, even during the sleep period, the mobile station must reliably track the amount of elapsed time to determine when the next slot occurs to permit receive components of the mobile station to power up in time to receive any paging signals transmitted to the mobile station within the slot. One solution to this problem is to operate a high frequency clock throughout the sleep period and to track the amount of elapsed time using the high frequency clock. This solution allows the sleep period to be very precisely tracked. However, considerable power is consumed operating the high frequency clock and optimal power savings therefore are not achieved during the sleep period.
Hence, it would be desirable to instead employ a low frequency, low power clock during the sleep period to reduce power consumption. However, clock signals provided by low frequency, low power clocks typically suffer from considerable frequency drift such that the amount of elapsed time during the sleep period cannot be precisely determined by counting cycles of the low power, low frequency clock. Frequency drift within a mobile station can be particularly significant if there are temperature variations within the mobile station caused by, for example, heat generated by the operation of components of the mobile station or by changes in ambient conditions. For example, during an extended telephone call, internal components of the mobile station may heat to 87 degrees Celsius. During an extended period of inactivity between telephone calls, the internal components may cool to an ambient temperature of, perhaps, 25 degrees Celsius. Moreover, if the user places the mobile telephone in either a very hot or very cold location, corresponding temperature changes within the mobile station may occur. Typical low power, low frequency clock signal generators are affected significantly by even slight changes in temperature and are even more strongly affected by the wide variations in temperature that can occur in a mobile telephone. Indeed, the amount of frequency drift within a typical low power, low frequency clock signal used in a mobile station may be so great that, if used by itself to calculate elapsed time within the sleep period, there is a significant risk that the mobile station will not be reactivated in time to power up components to detect a paging signal transmitted within a next paging slot. Accordingly, important paging signals maybe missed, possibly resulting in missed phone calls and the like. Thus the timing accuracy provided by a low frequency, low power clock signal is typically poor.
Another significant problem with using low frequency clock signals to track elapsed time within a sleep period is the relative lack of precision provided by the low frequency clock. The lack of precision can result in a considerable off-set between the initiation of the sleep period and a first counted cycle of the low frequency clock signal and also a considerable off-set between a last counted cycle of the low frequency clock and the actual end of the sleep period. More specifically, a counter is typically employed to count either rising edges or falling edges of the low frequency clock signal to track elapsed time within the sleep period and, once the number of cycles of the low frequency clock corresponding to the length of the sleep period have been counted, the high frequency clock is then re-activated. However, nearly an entire cycle of the low frequency clock may elapse between the beginning at the sleep period and the first edge of the low-frequency clock signal detected by the counter. The initial off-set can have a duration anywhere from zero to one full cycle of the low frequency clock or, in some systems, possibly even more. With conventional systems, it is not possible to determine the duration of the initial off-set. The uncertainty in the duration of the initial offset further increases the amount of error in the determination of elapsed time within the sleep period resulting in an even greater risk that the next paging slot will be missed. In an exemplary system wherein the high frequency clock operates at 9.68 megahertz and the sleep clock operates at 32 kilohertz, there are about 300 cycles of the high frequency clock within each cycle of the sleep clock. Therefore, even if the system can reliably compensate for frequency drift, the high frequency clock may still need to be activated as many as 300 cycles of the high frequency clock earlier than necessary to thereby account for the unknown duration of the intial offset. Also, because the re-activation of the high frequency clock at the end of the sleep period is synchronized with transitions in the low frequency clock, the degree of precision by which the high frequency clock can be re-activated is limited by the precision of the low frequency clock. For example, even if the system reliably and precisely determines that the correct duration of the sleep period is 853.44 cycles of the sleep clock, the system will need to re-activate the high frequency clock no later than the detected transition of the 853rd cycle and therefore will not properly account for the remaining fractional number of cycles, i.e. the remaining 0.44 cycles. With about 300 cycles of the high frequency clock occurring within each cycle of the sleep clock, in the example the high frequency clock is therefore turned on an additional 130 cycles earlier than necessary. In another example, if the correct duration of the sleep period is 853.99 cycles of the sleep mode clock, the high frequency clock will be turned on nearly 300 cycles earlier than necessary.
Hence, when using a low-frequency clock signal to track time during a sleep period, the mobile station is usually configured to return to an active mode well in advance of a next expected paging slot to thereby overcome possible timing errors cause by frequency drift in the low frequency clock and to compensate for the relative lack of precision in the low frequency clock. For example, if paging slots occur every 26.67 milliseconds, the mobile station may be programmed to activate the high frequency clock after only, for example, 25 milliseconds of sleep to ensure that the next paging slot is not missed. Hence, optimal power savings are not achieved.
One technique that has been proposed for compensating for timing errors inherent in low frequency, low power clock signal generators is to adapt a length of each sleep period based upon a timing accuracy of a previous sleep period. More specifically, if the mobile telephone wakes up too late within one sleep period to detect paging signals, the mobile station is adjusted to wake up earlier in the next sleep period. To determine whether a sleep period is too long or too short, the mobile station attempts to detect a unique word within a received paging signal, such as a message preamble which signifies the beginning of an assigned slot. If the unique word is not detected, the mobile station concludes that it woke up too late and therefore the sleep duration is decreased for the next sleep period. If the unique word was properly received, the mobile station either woke up on time or too early and the sleep duration is increased slightly for the next sleep period. One problem with the aforementioned technique is that it assumes that any failure to detect the unique word is the result of a timing error. However, there may be other reasons besides the duration of the sleep period that the unique word was not correctly received and demodulated, such as poor communication channel quality conditions. Moreover, even if failure to detect the unique word was the result of a timing error rather than other communication errors, the system still does not compensate for initial and final off-sets caused by the relative lack of precision in the low power, low frequency clock signal and therefore does not provide for optimal power savings.
A significant improvement is provided in U.S. patent application Ser. No. 09/134,808, entitled xe2x80x9cSynchronization of a Low Power Oscillator with a Reference Oscillator in a Wireless Communication Device Utilizing Slotted Paging,xe2x80x9d filed Aug. 14, 1998 and assigned to the assignee of the present invention. In the aforementioned patent application, timing errors are corrected without relying upon the failure to receive portions of transmitted signals. Rather, the system includes a frequency error estimation unit for directly estimating frequency drift in the low power, low frequency clock. In one example described in the patent application, frequency drift in the low frequency clock is determined by timing the low frequency clock using a high frequency clock during periods of time when the high frequency clock is active. For example, during each paging slot when the high frequency clock of the mobile station is operating, the frequency error in the low frequency clock is calculated based upon the high frequency clock. The system operates to synchronize the activation of the high frequency clock very precisely to transitions in the low frequency clock signal.
Although the system of the aforementioned patent application provides a significant improvement over systems which rely on the detection of unique words within signals transmitted to the mobile station, considerable room for improvement remains. For example, the aforementioned initial and final offsets are not taken into account. Accordingly, even with the improved system of the patent application, the high frequency clock, signal must usually be activated somewhat in advance of the next expected paging slot to account for remaining timing errors. Hence, optimal power savings are not achieved. It would be preferable to provide a system wherein the active mode high frequency clock is turned on as close as possible to the next paging slot to permit maximum power savings during the sleep period and it is to that end that aspects of the invention are primarily directed. In particular, it is desirable to provide a system which compensates for the aforementioned initial and final offsets to re-activate the high frequency clock to be re-activated based upon fractional portions of the low frequency clock, and particular aspects of the invention are directed to those ends.
In accordance with the invention, a device is provided for use in activating an active-mode clock following a sleep period for use within a mobile station wherein selected components of the mobile station operate using a sleep-mode clock during the sleep period and a faster active-mode clock during non-sleep periods. The device includes a wake-up estimation unit for estimating a wake up time using the sleep-mode clock. A compensation unit is provided for compensating for timing off-set errors in the estimated wake up time caused by differences in precision between the sleep-mode clock and the active-mode clock. An active-mode clock activation unit activates the active-mode clock at the compensated wake-up time to terminate the sleep mode.
In an exemplary embodiment, the mobile station operates in a mobile communications system employing slotted paging. The device includes a frequency drift compensation unit for compensating for an error in the estimated wake up time caused by frequency drift in the sleep-mode clock. By compensating for both frequency drift and timing off-sets, the active-mode clock is activated at a wake-up time closely in synchronization with a next paging slot and significant power savings are achieved as compared with systems wherein the active-mode clock must be activated well in advance of the next paging slot to compensate for possible timing errors.
In the exemplary embodiment, compensation for timing offsets and frequency drift is achieved using a transition-mode clock which is employed at both the beginning and the end of each sleep period. The transition-mode clock has a frequency substantially greater than that of the sleep mode clock. The transition-mode clock permits the device to conveniently compensate for both frequency drift errors and timing offset errors to permit the active-mode clock to be re-activated later in the sleep period. The transition-mode clock is deactivated shortly after the sleep period begins and is reactivated only slightly before the sleep period is due to end and hence very little power is consumed by the transition-mode clock. Moreover, because the transition-mode clock permits the components of the mobile station to be reactivated later within the sleep period, any increase in power required to operate the transition-mode clock is more than compensated for by power servings achieved by permitting a longer sleep period.
Method and apparatus embodiments of the invention are described.